1. Field of the Invention
The present invention relates to semiconductor devices, liquid crystal displays, and manufacturing methods thereof. More specifically, it relates to a semiconductor device provided with a thin-film field effect transistor and a conductive layer, a liquid crystal display, and manufacturing methods thereof
2. Description of the Background Art
Conventionally, one type of liquid crystal display is known utilizing a thin-film field effect transistor formed with low-temperature polysilicon. Such a liquid crystal display usually requires a capacitor for accumulation of electric charges. FIG. 13 shows a substrate with a thin-film field effect transistor of the liquid crystal display. FIG. 13 is a schematic cross a sectional view showing a conventional liquid crystal display. Referring to FIG. 13, the conventional liquid crystal display will be described.
In FIG. 13, the liquid crystal display has a driver circuit region and a display pixel region. On a glass substrate 101, a p channel thin-film field effect transistor 117 is located in the driver circuit region, whereas an n channel thin-film field effect transistor 118 and a storage capacitor 119 are located in the display pixel region.
In the driver circuit region, a base film 102 is formed on the glass substrate. Formed on base film 102 are source/drain regions 106a, 106b and a channel region 107 of p type thin-film field effect transistor 117, which are originally formed in the same layer of a polysilicon film as a semiconductor film. P type conductivity impurities are implanted into source/drain regions 106a, 106b. An insulating film 108 serving as a gate insulating film is formed on source/drain regions 106a, 106b and channel region 107. In the region above channel region 107, a gate electrode 109a is formed on insulating film 108. A protective film 111 is formed on gate electrode 109. Contact holes 112a, 112b are formed opposite source/drain regions 106a, 106b by partially etching protective film 111 and insulative film 108. Electrodes 113a, 113b are formed in contact holes 112a, 112b, extending on the surface of protective film 111. An insulative film 114 is formed on electrodes 113a, 113b and protective film 111.
In the display pixel region of the liquid crystal display, base film 102 is formed on glass substrate 101. Formed on base film 102 are source/drain regions 104a, 104b and a channel region 105 of an n channel thin-film field effect transistor 118, which are originally formed in the same layer of a polysilicon film as a semiconductor film. In addition, on base film 102, a lower electrode 103 of storage capacitor 119 is formed by the same layer of semiconductor film as that of source/drain regions 104a, 104b and channel region 105. Insulative film 108 is formed on source/drain regions 104a, 104b, channel region 105, and lower electrode 103. Insulative film 108 has portions respectively serving as a gate insulating film of n channel thin-film field effect transistor 118 and a dielectric film of storage capacitor 119. Namely, insulative film 108 on channel region 105 serves as a gate insulative film, whereas insulative film 108 on lower electrode 103 serves as a dielectric film. In the region on channel region 105, a gate electrode 109b is formed on insulative film 108. In the region on lower electrode 103, a common electrode 110 is formed on insulative film 108 serving as a dielectric film. A protective film 111 is formed on gate electrode 109b and common electrode 110. Protective film 111 and insulative mm 108 are partially etched and removed to form contact holes 112c to 112e. Electrodes 113c to 113e are respectively formed in contact holes 112c to 112e to extend on the surface of protective film 111. An insulative film 114 is formed on electrodes 113c to 113e and protective film 111. Thereafter, a transparent electrode and the like are formed in the display pixel region to manufacture a liquid crystal display in a conventional process.
As described above, the gate insulative film of p and n channel thin-film field effect transistors 117 and 118 and the dielectric film of storage capacitor 119 are formed by the same layer, i.e., insulative film 108. Thus, the manufacturing process of the liquid crystal display is simplified
When a coplanar thin-film field effect transistor is used, lower electrode 103 of storage capacitor 119 is formed by implanting impurities in the same semiconductor film as that forming channel regions 107 and 105 of p and n channel thin-film field effect transistors 117 and 118. This is because the thin-film field effect transistor is extremely sensitive to metal impurities in a process up to formation of the gate insulative film and it is substantially difficult to form a metal electrode below the gate insulative film.
FIGS. 14 to 16 are schematic cross sectional views shown in conjunction with the manufacturing method of the liquid crystal display in FIG. 13. Referring to FIGS. 14 to 16, the manufacturing method of the liquid crystal display will be described.
Referring first to FIG. 14, a base film 102 is formed on a glass substrate 101 by a general method such as PECVD (Plasma Enhanced Chemical Vapor Deposition). A 2-layer film of silicon nitride and oxide films can be used as base film 102. An amorphous silicon film is formed on base film 102. The amorphous silicon film to be channel regions of p and n type thin-film field effect transistors 117 and 118 are annealed by an excimer laser to form a polysilicon film. Thereafter, a resist film is formed on the thus-formed polysilicon film. Polysilicon films 124a to 124c are formed as semiconductor films shown in FIG. 14 by using the resist film as a mask for dry etching. Then, the resist film is removed.
Successively, phosphorus (P) ions, n type conductivity impurities are implanted in polysilicon film 124c to be a lower electrode of storage capacitor 119. The phosphorus ions are selectively implanted in polysilicon film 124c, so that a resist film 125 is formed to cover polysilicon films 124a and 124b in a region excluding polysilicon film 124c. Phosphorus ions 129 are implanted in polysilicon film 124c by using resist film 125 as a mask to form a lower electrode 103. The implantation of phosphorus ions 129 forms on resist film 125 a layer 126 of which a property has been changed by the implantation of phosphorus ions (hereinafter referred to as a changed layer 126). On the other hand, the layer underlying resist film 125 has not been affected by the implantation of phosphorus ions 129. Thus, resist film 125 includes two layers, changed layer 126 and unchanged layer 127.
Thereafter, resist film 125 is removed. Note that if a usual stripper is used to remove changed layer 126, the removal requires a considerable time or a changed layer 126 cannot be removed. Thus, plasma ashing with an oxygen plasma is used for the removal of changed layer 126. More specifically, changed layer 126 is removed by directing oxygen plasma 133 to the surface of changed layer 126 of resist film 125, as shown in FIG. 16. After changed layer 126 is removed, a usual stripper is used to remove unchanged layer 127.
Subsequently, insulative film 108 (see FIG. 13) serving as a gate insulative film and a dielectric film of storage capacitor 119 is formed. For example, a silicon oxide film formed by TEOS PECVD may be used as insulative film 108. A chrome film is formed on insulative film 108 by sputtering. A resist film is formed on the chrome film. The chrome film is partially removed by etching using the resist film as a mask, so that gate electrodes 109a, 109b and common electrode 110 (see FIG. 13) are formed. Storage capacitor 119 is formed by common electrode 110, lower electrode 103, and insulative film 108. Then, phosphorus ions producing n type conductivity are implanted into source/drain regions 104a, 104b. P type conductive impurities such as boron (B) ions are implanted into source/drain regions 106a, 106b. This forms p and n channel thin-film field effect transistors 117 and 118.
Then, protective film 111 (see FIG. 13) is formed on gate electrodes 109a, 109b and common electrode 110. A silicon oxide film formed by TEOS CVD may be used as protective film 111. Thereafter, activation is performed by annealing at a temperature of 40xc2x0 C. A resist film is formed on protective film 111. Protective film 111 and insulative film 108 are partially removed using the resist film as a mask, so that contact holes 112a to 112e are formed. The resist mm is then removed. A chrome film is formed in contact holes 112a to 112e, extending on the surface of protective film 111. The thickness of the chrome film is 100 nm. An aluminum containing alloy film is formed on the chrome film by sputtering. The thickness of the aluminum containing alloy film is 400 nm. A resist film is formed on the aluminum containing alloy film. The aluminum containing alloy film and chrome film are anisotropically etched using the resist film as a mask to form electrodes 113a to 113e (see FIG. 13). The resist film is then removed. Electrodes 13a to 13e are formed by the above mentioned chrome film and the aluminum containing alloy film.
The properties of the thin-film field effect transistor are enhanced and stabilized by hydrogenation of channel regions 105 and 107 with a hydrogen plasma. An insulative film 114 is formed on electrodes 113a to 113e. For example, a silicon nitride film may be used as insulative film 114. Thus, a structure shown in FIG. 13 is obtained.
In the driver circuit region, an n channel thin-film field effect transistor is formed by the above described method in addition to p channel thin-film field effect transistor 117 to form a driver circuit. In the display pixel region, a display pixel is formed by electrically connecting n channel thin-film field effect transistor 118 to a transparent electrode, which is separately formed. Further, the glass substrate with these elements as a semiconductor device is applied to another glass substrate on which a color filter, opposed electrode and the like are formed. Then, a prescribed process is performed including pouring a liquid crystal into the gap between the glass substrates and sealing the gap to provide the liquid crystal display.
As stated above, in the conventional method of manufacturing the liquid crystal display, changed layer 126, which has been changed by implantation of phosphorus ions 129, is formed on resist film 125 when implanting phosphorus ions 129 in the step shown in FIG. 15. For removal of changed layer 126, an oxygen plasma process using an oxygen plasma 113 is performed as shown in FIG. 16.
However, the following problem arises in the oxygen plasma process. Namely, impurities such as phosphorus ions implanted in changed layer 126 of resist film 125 flow to an ambient as changed layer 126 is removed in the oxygen plasma process (an ashing step). Some of the flowing phosphorus ions are again implanted as impurities 136 into polysilicon films 124a, 124b to be channel regions 105, 107 due to an electric field generated in the oxygen plasma process. The re-implanted impurities 136 in polysilicon films 124a, 124b cause the impurity concentrations of channel regions 105, 107 to be higher than a set value. Consequently, a study of the inventors has revealed that the threshold voltages of p and n channel thin-film field effect transistors 117 and 118 change with the impurity concentrations of channel regions 105, 107. The change in threshold voltage will be explained with reference to FIG. 17.
FIG. 17 is a graph showing a relationship between the impurity concentration in the channel region of the thin-film field effect transistor and an amount of change in threshold voltage. FIG. 17 relates to the case where n type conductivity impurities (for example phosphorus ions) are implanted into the channel region of the p channel thin-film field effect transistor. As is apparent from FIG. 17, once the impurity concentration exceeds 1016 atoms/cm3, there is a sharp rise in threshold voltage, exceeding the set value.
Implantation of the n type conductivity impurities into the channel region of n channel thin-film field effect transistor decreases the threshold voltage. Absolute values of amounts of decrease in threshold voltage are depicted to provide a graph similar to that shown in FIG. 17.
If the threshold voltage of the thin-film field effect transistor changes, the operation of a driver circuit portion utilizing such a thin-film field effect transistor becomes unstable in the liquid crystal display, thereby causing defective products. As a result, yield of liquid crystal displays disadvantageously decreases.
Several methods are contemplated to solve the aforementioned problem. For example, as shown in FIG. 18, insulative films 108 serving as a gate insulative film and dielectric film are preliminary formed on polysilicon films 124a, 124b and lower electrode 103. FIG. 18 is a schematic cross sectional view shown in conjunction with another manufacturing method of a liquid crystal display. As shown in FIG. 18, if insulative film 108 is preliminary formed, in a plasma ashing process using an oxygen plasma as shown in FIG. 16, insulative film 108 is formed on polysilicon films 124a, 124b. Insulative film 108 serves as a protective film for preventing re-implantation of impurities 139 (see FIG. 16) into polysilicon films 124a, 124b. Thus, re-implantation of the impurities such as phosphorus ions into polysilicon films 124a, 124b to be channel regions 105, 107 is prevented.
In this case, however, in the step of implanting phosphorus ions shown in conjunction with FIG. 18, phosphorus ions 129 must be implanted into lower electrode 103 positioned below insulative film 108. This necessitates a greater implantation energy (an acceleration energy) of phosphorus ions 129. If such phosphorus ions accelerated by the great energy are to be implanted into lower electrode 103, insulative film 108 serving as a dielectric film may be damaged by the ion implantation or the conductive impurities such as phosphorus ions 129 are left in insulative film 108. If insulative film 108 is damaged, the breakdown voltage of insulative film 108 is decreased. As a result, the durability and reliability of storage capacitor 119 decreases. Further, if the acceleration energy of phosphorus ions 129 is high as described above, changed layer 126 of resist film 125 expands and its properties are changed to a larger extent. Thus, resist film 125 cannot be well removed in the step of removing resist film 125. Such poor removal of resist film 125 prevents formation of p and n channel thin-film field effect transistors 117 and 118 in a prescribed manner, thereby resulting in poor operation of the thin-film field effect transistor. Consequently, the yield of liquid crystal displays decreases.
In addition, generally, impurities of about 1020 atoms/cm3 must be implanted into lower electrode 103. In this case, a certain amount of impurities are implanted also into insulative film 108 positioned on lower electrode 103. The concentration of the impurities to be implanted into insulative film 108 would be at least about 1018 atoms/cm3, judging from the data shown in FIG. 19. FIG. 19 is a graph showing a relationship between a depth of a target material from an implantation surface and a boron concentration in that position when boron ions are used as impurity ions and implanted into a target material of silicon. The graph of FIG. 19 shows a result when an implantation energy of the boron ions is varied. Referring to FIG. 19, if, for example, the boron ions are implanted under a condition of the implantation energy of 4.8xc3x9710xe2x88x9215J (30 keV), the boron concentration in the position about 0.1 xcexcm from the surface of the implantation surface is 1020 atoms/cm3. Then, the boron concentration is about 1019 atoms/cm3 in the surface layer (in the position with a depth of 0 xcexcm) of silicon. If the implantation energy of the boron ions is varied under a condition of the boron concentration of about 1020 atoms/cm3in the region with the highest boron concentration, the boron concentration in the surface layer of the target material is at least about 1018 atoms/cm3.
Judging from the above, phosphorus ions 129 would be included in a concentration of about 1018 atoms/cm3 in insulative film 108 on the upper surface (corresponding to the surface layer) of lower electrode 103, which is a target material into which phosphorus ions 129 of the conductive impurities are implanted. Namely, when the step of implanting phosphorus ions 129 is performed with insulative film 108 preliminary formed as a protective film, impurity ions of about 1018 atoms/cm3 would be implanted into insulative film 108 on lower electrode 103. Such residual conductive impurities in the dielectric film causes a decrease in durability and reliability of storage capacitor 119. The data of FIG. 19 is derived from xe2x80x9cUltra LSI Process Data Handbook,xe2x80x9d SCIENCE FORUM, Mar. 31, 1990, p232. In FIG. 19, 50 keV, 70 keV, 100 keV, and 200 keV respectively represent 8.0xc3x971015J, 1.1xc3x971014J, 1.6xc3x971014J, and 3.2xc3x971014J.
Another possible method of solving the problem will be described below, which is associated with the re-implantation of impurities such as phosphorus ions into polysilicon films 214a, 124b to be channel regions 105, 107. FIG. 20 is a schematic cross sectional view showing another exemplary liquid crystal display manufactured by the method described below. Referring to FIG. 20, the liquid crystal display basically has the same structure as the conventional liquid crystal display shown in FIG. 13. However, an insulative film serving as a gate insulative film and a dielectric film of storage capacitor 119 has a 2-layer structure of lower and upper insulative films 137 and 138.
FIGS. 21 and 22 are schematic cross sectional views shown in conjunction with the manufacturing method of the liquid crystal display of FIG. 20. The manufacturing method of the semiconductor device will be described with reference to FIGS. 21 and 22.
The step shown in FIG. 21 is basically the same as that of FIG. 18. Note that formed on polysilicon films 124a, 124b and lower electrode 103 is lower insulative film 137 as a protective film, which is smaller than insulative film 108 in thickness. After phosphorus ions 129 are implanted with lower insulative film 137 as such a thin protective film formed, resist film 125 is removed by, for example, plasma ashing. Thereafter, upper insulative film 138 is formed on lower insulative film 137 as shown in FIG. 22. A total thickness of lower and upper insulative films 137 and 138 is adjusted to be approximately equal to a thickness required for the gate insulative film of p and n type thin-film field effect transistors 117 and 118.
As shown in FIG. 21, since lower insulative film 137 is used as a protective film which is smaller than insulative film 108 of FIG. 18 in thickness, the implantation energy of phosphorus ions 129 is lower than in the step shown in FIG. 18. As a result, damage to insulative film 108 caused by the implantation of the phosphorus ions in the step of FIG. 18 can be, to some extent, reduced.
However, if the step shown in FIGS. 21 and 22 are employed, the greater number of steps are necessary than in the manufacturing method of FIG. 18. Further, if the gate insulative film has two layers as shown in FIG. 22, it is known that the breakdown voltage and reliability become lower than when the gate insulative film has a single film. As a result, yield of liquid crystal display decreases due to reduction in breakdown voltage and reliability of p and n channel thin-film field effect transistors 117 and 118 as well as storage capacitor 119.
As described above, in the semiconductor device provided with a conductive layer formed by the same layer as the channel region of the thin-film field effect transistor, conventionally, it is difficult to stabilize the threshold voltage by preventing re-implantation of impurities into the channel region of the thin-film field effect transistor while ensuring sufficient durability and reliability of the storage capacitor including lower electrode 103 as the conductive layer.
One object of the present invention is to provide a semiconductor device provided with a thin-film field effect transistor and a conductive layer and manufacturing method thereof capable of stabilizing a threshold voltage of the thin-film field effect transistor while enhancing durability and reliability of the capacitor of which conductive layer is used as an electrode.
Another object of the present invention is to provide a liquid crystal display provided with a thin-film field effect transistor and a conductive layer and a manufacturing method thereof capable of stabilizing a threshold voltage of the thin-film field electrode transistor while enhancing durability and reliability of a capacitor of which conductive layer is used as an electrode.
A semiconductor device according to one aspect of the present invention is provided with a thin-film field effect transistor and a conductive layer and also includes a substrate, a semiconductor film, a conductive layer, and a dielectric film. The semiconductor film includes a channel region of the thin-film field effect transistor formed on the substrate. The conductive layer is originally formed on the substrate in the same layer as the semiconductor film. The dielectric film is formed on the conductive layer. A conductive impurity concentration of the channel region is at most 1016 atoms/cm3. A conductive impurity concentration of the dielectric film is at most 1017 atoms/cm3.
As stated above, since the conductive impurity concentration of the channel region of the thin-film field effect transistor is at most 1016 atoms/cm3, the threshold voltage of the thin-film field effect transistor would not change.
In addition, since the conductive impurity concentration of the dielectric film is at most 1017 atoms/cm3, reduction in breakdown voltage and reliability of the dielectric film is prevented. Thus, if a capacitor is formed having as one electrode a conductive layer and the other electrode as one conductive layer opposite to the conductive layer of one electrode through the dielectric film, durability and reliability of the capacitor can be enhanced. When the conductive impurities are implanted into the conductive layer through the dielectric film after formation of the dielectric film, the conductive impurity concentration of the dielectric film becomes at least 1018 atoms/cm3 as described above. Thus, if the conductive impurity concentration of the dielectric film is to be at most 1017 atoms/cm3, the conductive impurities must be implanted into the conductive layer with no dielectric film formed. Namely, damage to the dielectric film due to implantation of the conductive impurities can be prevented.
In the semiconductor device according to the above described one aspect, the thin-film field effect transistor has source and drain regions of a first conductivity type. Preferably, the conductive impurities have a second conductivity type opposite to the first conductivity type.
Here, if the conductive impurities in the channel region is of the second conductivity type, the threshold voltage of the thin-film field effect transistor increases with increase in conductive impurity concentration. With the conductive impurity concentration of the channel region set in a range defined by the present invention, it is ensured that the threshold voltage of the thin-film field effect transistor would not exceed a set value.
In the semiconductor device according to the above mentioned one aspect, the thin-film field effect transistor has source and drain regions of a first conductivity type, and the conductive impurities may have the first conductivity type.
In this case, it is ensured that the threshold voltage of the thin-film field effect transistor would not fall below a set value because of the presence of the conductive impurities.
A liquid crystal display according to another aspect of the present invention is provided with a semiconductor device according to the above mentioned one aspect.
Thus, the thin-film field effect transistor is applied to a switching element in the display pixel region of the liquid crystal display and the conductive layer is applied to the electrode of the storage capacitor of the pixel to easily stabilized the threshold voltage of the thin-film field effect transistor in the display pixel region while enhancing durability and reliability of the storage capacitor. As a result, a display property of the liquid crystal display can be enhanced.
In the manufacturing method of the semiconductor device provided with the thin-film field effect transistor and the conductive layer according to another aspect of the present invention, a semiconductor film is formed on the substrate. A first resist film is formed on the semiconductor film. The semiconductor film is etched by using the first resist film as a mask to form first semiconductor film to be a channel region of the thin-film field effect transistor and second semiconductor film. The first resist film is removed. A second resist film is formed at least on the first semiconductor film in a region excluding the second semiconductor film. Conductive impurities are implanted into the second semiconductor film using the second resist film as a mask to form a conductive layer. The second resist film is removed by ultraviolet light irradiation or wet etching. A dielectric film is formed on the conductive layer after formation of the conductive layer.
In this method, plasma ashing using for example an oxygen plasma is not employed for removal of the second resist film, so that it is ensured that the conductive impurities left in the resist would not be implanted into the first semiconductor film to be the channel region of the thin-film field effect transistor. This prevents unnecessary increase in the conductive impurity concentration of the first semiconductor film, whereby the threshold voltage of the thin-film field effect transistor having the first semiconductor film as the channel region would not change.
Since the dielectric film is formed after formation of the conductive layer, damage to the dielectric film due to implantation of the conductive impurities can be prevented. As a result, durability and reliability of the dielectric film can be enhanced. This enhances breakdown voltage and reliability of a capacitor or the like formed by using the dielectric film.
In the manufacturing method of the semiconductor device according to the above mentioned another aspect, the second resist film may include a lower layer portion and an upper layer portion thereon. The step of removing the second resist film preferably includes a step of removing the upper layer portion by ultraviolet light irradiation and a step of removing the lower layer portion by wet etching.
In this case, it is ensured that the upper layer portion including a portion of which property has been changed due to implantation of the conductive impurities can be removed by ultraviolet light irradiation and a lower layer portion can be removed in a relatively short time by wet etching. This avoids the problem associated with the left portion of the changed layer and shortens the time required for manufacturing the semiconductor device.
In the manufacturing method of the semiconductor device according to the above mentioned another aspect, prior to the step of forming the second resist film, preferably, a protective film is formed on the first semiconductor film and the protective film is removed after the step of removing the second resist film. The step of removing the second resist film is preferably performed with the protective film left.
In this case, it is ensured that the protective film prevents implantation of the conductive impurities left in the second resist film into the first semiconductor film in the step of removing the second resist film. As a result, unnecessary increase in the conductive impurity concentration of the first semiconductor film can be prevented.
In the manufacturing method of a semiconductor device provided with a thin-film field effect transistor and a conductive layer according to another aspect of the present invention, a semiconductor film is formed on a substrate. A first resist film is formed on the semiconductor film. The semiconductor film is etched by using the first resist film as a mask to form first and second semiconductor films to be channel regions of the thin-film field effect transistor. A protective film is formed on the first semiconductor film. A second resist film is formed at least on the protective film in a region excluding the second semiconductor film. Conductive impurities are implanted into the second semiconductor film using the second resist film as a mask to form a conductive layer. The second resist film is removed with the protective film left after formation of the conductive layer. The protective film is removed. After the removal of the protective film, a dielectric film is formed on the conductive layer.
In this method, it is ensured that implantation of the conductive impurities left in the second resist film into the first protective film can be prevented because of the presence of the protective film in the step of removing the second resist film. As a result, unnecessary increase in the conductive impurity concentration of the first semiconductor film can be avoided. Thus, it is ensured that the conductive impurity concentration of the channel region of the thin-film field effect transistor does not exceed a prescribed value, whereby the threshold voltage of the thin-film field effect transistor would not change.
Further, subsequent to the removal of the protective film, i.e., after implantation of the conductive impurities, damage to the dielectric film due to implantation of the conductive impurities is prevented because the dielectric film is formed on the conductive layer. Accordingly, reduction in breakdown voltage or reliability of the dielectric film caused by the damage to the dielectric film is prevented. Thus, durability and reliability of an element such as a capacitor with the dielectric film can be enhanced.
In the manufacturing method of a semiconductor device provided with a thin-film field effect transistor and a conductive layer according to still another aspect of the present invention, a semiconductor film is formed on a substrate. A coating to be a protective film is formed on the semiconductor film. A first resist film is formed on the coating. The semiconductor film and the coating are etched and removed by using the first resist film as a mask to form first semiconductor film to be a channel region of the thin-film field effect transistor, second semiconductor film and a protective film on the first and second semiconductor films. A second resist film is formed at least one the protective film in a region excluding the second semiconductor film. Conductive impurities are implanted into the second semiconductor film by using the second resist film as a mask to form a conductive layer. After the step of forming the conductive layer, the second resist film is removed with the protective film still left. Then the protective film is removed. After the removal of the protective film, a dielectric film is formed on the conductive layer.
In this method, the second resist film is removed with the protective film still left, implantation of the conductive impurities left in the second resist film into the first semiconductor film can be avoided. Thus, unnecessary increase in the conductive impurity concentration of the channel region of the thin-film field effect transistor can be prevented.
Further, the dielectric film is formed subsequent to the removal of the protective film, i.e., after implantation of the conductive impurities, damage to the dielectric film caused by the implantation of the conductive impurities is avoided. Thus, the quality of the dielectric film would not be impaired, whereby breakdown voltage and reliability of the dielectric film can enhanced. As a result, durability and reliability of an element such as a capacitor with the conductive layer and the dielectric film can be enhanced.
In the manufacturing method of a semiconductor device according to the above described another aspect or still another aspect, the step of removing the second resist film preferably includes a step of removing an upper layer portion including an upper surface of the second resist film by plasma ashing, and a step of removing a lower layer portion of the second resist film under the upper layer portion by wet etching.
In this case, it is ensured that the upper layer portion including a region (changed layer) of which property has been changed by the implantation of the conductive impurities is removed by plasma ashing. At the same time, since the lower layer portion is removed by wet etching, the problem associated with the left portion of the changed layer is avoided. In addition, wet etching shortens the manufacturing time.
In the manufacturing method of the semiconductor device according to the above described another aspect or still another aspect, the upper layer portion of the second resist film is preferably a changed layer which has been subjected to implantation of conductive impurities.
In this case, it is ensured that the changed layer is removed by plasma ashing. Thus, a defect would not be caused to the structure of the thin-film field effect transistor due to the left unchanged layer.
In the manufacturing method of the semiconductor device according to the above described another aspect or on of still another aspects, preferably, the protective film is an oxide film or a nitride film.
In this case, the protective film can easily be formed by CVD (Chemical Vapor Deposition), thermal oxidation, or ultraviolet light irradiation, which has conventionally been employed for the manufacturer of the semiconductor devices. This eliminates the need for an investment involving introduction of a manufacturing device for forming the protective film. Thus, the manufacturing cost of the semiconductor device would not be increased.
In the manufacturing method of the semiconductor device according to the above described another aspect or one of still another aspects, preferably, the protective film is formed by CVD or sputtering.
In this case, by adjusting conditions of the CVD or the like, the protective film can easily be adjusted to have an arbitrary thickness.
If CVD is used in the step of forming the first semiconductor device, formation of the continuous protective film simplifies the process.
In the manufacturing method of the semiconductor device according to the above described another aspect or one of still another aspects, the protective film may be formed by ultraviolet light irradiation.
In this case, the protective film can be formed by a simple and time-saving process such as ultraviolet light irradiation, whereby the manufacturing process is simplified and completed in a shorter time.
The manufacturing method of the liquid crystal display according to still another aspect of the present invention employs the manufacturing method of the semiconductor device according to the above described another aspect or one of still another aspects.
Thus, if the thin-film field effect transistor is used as a circuit element in the display pixel region of the liquid crystal display and the conductive layer is used as an electrode of a storage capacitor in the display pixel region, the threshold voltage of the thin-film field effect transistor in the display pixel region is stabilized and durability and reliability of the capacitor can be enhanced. This enhances the display property of the liquid crystal display.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.